1. Field of the Invention
The present invention relates generally to electron guns for cathode ray tubes (CRTs) and more specifically to electron guns of the type used with monochrome tubes suitable for projection-type television receivers.
2. Discussion of the Related Art
In order to produce a high resolution image on a CRT raster, the electron beam which is swept over the phosphor screen must be well focused and of small size. Spherical aberrations inherent in the electron-optical lenses of the electron gun which forms and emits the electron beam can create distorted beam "spots" at the screen which become larger-than-optimal size. In order to reduce spherical aberrations in the gun and, therefore, achieve a smaller or better focused beam spot, lenses with larger apparent apertures are desirable within the electron gun structure. However, lens sizes are limited by the size of the CRT neck which the gun must fit within. Past teachings which expound upon and address these problems include U.S. Pat. No. 4,904,898 to Penird et al.; U.S. Pat. No. 4,271,374, to Kimura; U.S. Pat. No. 4,649,318 to Kukuchi et al.; and U.S. Pat. No. 4,728,846 to Yasuda. Each of these references illustrate what is herein called an "immersion" lens, where the lower potential grid of the accelerating portion of the main lens is fitted partially within the higher potential grid in order to achieve a larger apparent lens aperture with reduced spherical aberration. Kimura, Penird et al., and Kukuchi et al. describe einzel-type guns, while Yasuda shows a bipotential type gun. All of the cited references show an immersion lens for the accelerating portion of the main lens, which is the G3-G4 gap in the bipotential gun of Yasuda and the G4-G5 gap of the einzel guns of the others.
A further consideration for the operation of such guns, as recognized in the Penird et al. patent, is that the electron guns in projection-type CRTs are operated at higher voltages than direct-view CRTs in order to provide adequate brightness. Therefore, high voltage stability of the gun and the prevention of arcing between the grids, becomes of increasing concern while still maintaining an adequate spot size. The cited references are somewhat elaborate structurally, thus requiring intensive effort in forming the grids and/or assembling them into the gun to prevent artifacts such as burrs and microcracks on the smooth surfaces of the grids which would lead to high voltage instability. Further effort must be taken with the cited designs to prevent axial misalignments of the grids in relation to one another which may skew the lensing action of the grids and result in beam asymmetries or large spot sizes.
Certain structural improvements to the gun designs of the prior art are, therefore, desired in order to decrease spot size for high resolution while gaining high voltage stability and ease of manufacture of the gun.